The present invention is directed to a method for removing and recovering catalyst components from organic reaction mixtures and, more specifically, to a method for removing and recovering both metal catalyst components and substantially water-soluble solvents from organic reaction mixtures comprising carbonylation reaction products.
Aromatic carbonates find utility, inter alia, as intermediates in the preparation of polycarbonates. For example, a popular method of polycarbonate preparation is the melt transesterification of aromatic carbonates with bisphenols.
Various methods for preparing aromatic carbonates have been previously described in the literature and/or utilized by industry. A method that has enjoyed substantial popularity in the literature involves the direct carbonylation of aromatic hydroxy compounds with carbon monoxide and oxygen catalyzed by at least one Group 8, 9, or 10 metal source. Further refinements to the carbonylation catalyst composition include the identification of co-catalysts.
The utility of the carbonylation process is strongly dependent on the number of moles of aromatic carbonate produced per mole of metal catalyst utilized (i.e. xe2x80x9ccatalyst turnover number or xe2x80x9cTONxe2x80x9dxe2x80x9d). Consequently, much work has been directed to the identification of efficacious process and catalyst variations that increase catalyst turnover and yield of aromatic carbonate. For example, in U.S. Pat. No. 5,498,789 a catalyst system for carbonylation has been disclosed which consists of a palladium catalyst, lead compound, and an organic bromide. GB 2311777A discloses a catalyst system which comprises a palladium catalyst, a lead compound, a cobalt compound, and a halide. More efficient catalyst systems for carbonylation are reported, for example, in U.S. Pat. No. 6,114,564, 6,172,254, and 6,180,812, all assigned to the assignee of the present invention, in which catalyst systems may comprise alkali metal halides and an activating solvent (sometimes known as a xe2x80x9cpromoter compoundxe2x80x9d).
Recovery and reuse of all catalyst and recyclable components from a carbonylation reaction are imperative if a process to prepare aromatic carbonates is to be economically viable and environmentally safe. In particular, all metal components from a carbonylation reaction must be recovered and recycled efficiently.
One possible method of recovery of metallic or other catalyst components comprises an aqueous extraction, for example, as is disclosed in U.S. Pat. Nos. 5,981,788 and 6,090,737, assigned to the assignee of the present invention. Although metals may be recovered by such extraction processes, nevertheless a portion of metal components is often not removed and recovered from a carbonylation reaction mixture. Recovery may be complicated by the fact that metals are often present in very low concentrations. Also, metals such as palladium and other metal co-catalysts are often present in a mixture of different oxidation states and physical phases at the end of a carbonylation reaction, making segregation of elemental species from oxidized species more likely, and thus requiring further complexity in recovery and recycle schemes. In particular insoluble metal species may be left behind in the reactor when a carbonylation reaction mixture comprising soluble metal species is removed from a reactor. It would be beneficial to remove the metal components at an early stage from a completed carbonylation reaction mixture, for example to prevent metal-promoted decomposition of the aromatic carbonate during product recovery and purification. Also, it would be desirable to have the metals all removed at one time rather than a portion in separate steps of recovery processes.
Also, when an activating solvent is present in a carbonylation reaction mixture, then the activating solvent must be recovered and recycled efficiently. Often, it is desirable to retain an activating solvent in an organic phase until it can be separated, for example by distillation. Since separation by distillation is often a high temperature process, decomposition and transesterification of aromatic carbonate product may occur, for example, if other catalyst components such as metals are present during a distillative separation of activating solvent and product. As discussed above, one possible method of removal of metallic or other catalyst components is by an aqueous extraction. In this case it is often desirable that an activating solvent remain in an organic phase during an aqueous extraction process so that it does not finally have to be separated from an aqueous stream containing other catalyst components such as metals. Many disclosed activating solvents such as polyethers tend to have a high solubility in water. A problem to be solved is to devise a method for efficient recovery of water-soluble activating solvents from complex carbonylation reaction mixtures without the necessity of separating such activating solvents from an aqueous stream. A more general problem to be solved is to devise a method of removing and recovering both metal species and activating solvent from a carbonylation reaction mixture is an integrated process.
After diligent experimentation the present inventors have discovered a method for removal and recovery of both metal species and activating solvent from complex carbonylation reaction mixtures with excellent efficiency. Thus, in one embodiment, the present invention provides a method for recovering a substantially water-soluble solvent and at least one metal from an organic reaction mixture comprising at least about 35% by weight aromatic hydroxy compound, which comprises the steps of: (i) contacting a reaction mixture at least once with aqueous acid, (ii) mixing the organic and aqueous phases in the presence of an oxygen source, (iii) separating the organic and aqueous phases wherein said solvent remains substantially in the organic phase; (iv) recovering metal species from the aqueous phase; and (v) recovering said solvent from the organic phase.
Various other features, aspects, and advantages of the present invention will become more apparent with reference to the following description and appended claims.